In nuclear reactors such as pressurized water nuclear reactors, the fluid for cooling the fuel assemblies constituting the core of the reactor is circulated in the cooling circuit or primary circuit by motor-pumps comprising a helicocentrifugal pump and a drive motor for this pump.
Such a centrifugal pump includes a volute constituted by an enclosure generally welded to the pipes of the primary circuit, in which openings are formed for the entry and exit of the cooling fluid in the pump.
A dismountable suction pipe extends the opening for entry into the side of the volute and a rotor mounted on a shaft connected to the drive motor is placed inside the volute, in extension of the suction pipe for the circulation of the cooling fluid. This rotor is constituted by an impeller wheel which is itself surrounded by a fixed diffuser bearing guide fins for the fluid and fastened at one of its ends to the suction pipe. This diffuser is generally connected rigidly and permanently with the volute either directly or only through the suction pipe.
An opening is provided in the volute to permit the passage of the rotor at the time of assembling the pump, this opening being then closed by a fluid-tight passage enabling the traversal of the shaft of the rotor between the inside of the volute and the drive latter. However, this solution leads to an unfavorable hydraulic shape and requires a relatively large diameter of the opening of the volute.
If a radial shape of the diffuser more favorable from the hydraulic point of view is adopted, an upper axial opening of the volute must be provided which is still larger, and this presents drawbacks which have been mentioned above.